This invention relates to a process for making periodic profiles, in particular a to a method of fabricating grating structures, especially blazed grating structures in photoresist material.
Grating structures, which are structures having a periodic profile in one particular cross section, can be fabricated using a variety of techniques. A convenient and well known method of producing a grating structure is to expose a layer of photo-resist material to a pattern of light of a periodically varying intensity. For a positive photoresist material, any area which is exposed to light can be removed by a development process whilst for a negative photoresist material any area which is not exposed to light can be removed by a development process. Photoresist materials are generally only photosensitive to radiation in a certain range.
One method of producing light of a periodically varying intensity is to form an interference pattern. The exposure of a photoresist layer to such an interference pattern, which can be produced by any number of interferometric techniques that are well known to a person skilled in the art, allows a range of symmetric grating profiles to be produced. However, by the nature of this technique, it is difficult to obtain a high degree of grating asymmetry uniformly over a large area. Obtaining short pitch gratings with large groove depths is also difficult using this technique. To obtain grating profiles other than sinusoidal requires over-exposure of the layer, and the level of background radiation associated with the interference pattern may then expose the whole photoresist layer.
A greater range of grating profiles may be obtained using hard contact photolithography, in which a chrome mask is placed in intimate contact (within 0.2 xcexcm) with the photo-resist coated substrate. This approach allows asymmetric grating profiles to be attained but, because of the requirement for the mask to be in intimate contact with the substrate, it is difficult to realise asymmetric gratings over a large area. Additionally, the minimum pitch of grating which can be produced using this technique is limited by the resolution of chrome mask fabrication. The highest resolution chrome mask which can be readily fabricated is of approximately 0.8 xcexcm pitch, and such masks are generally fragile, therefore having a short useful lifetime, and may produce unwanted diffraction effects.
It is also possible to fabricate grating structures using ruling techniques, but these tend to be prohibitively expensive.
It is an object of the present invention to provide an inexpensive method of making gratings that mitigates some of the disadvantages, as described above, that are associated with conventional methods of grating fabrication.
According to this invention, a process for making a periodic profile in a photoresist layer comprises the steps of taking a first layer of photoresist material which will react to radiation of a first wavelength and which has deposited on it a second layer of photoresist material which will react to radiation of a second wavelength and wherein the first photoresist material is substantially insoluble in the second solvent and wherein the layer of second photoresist material is substantially opaque to the radiation of the first wavelength, exposing the layer of second photoresist material to a pattern of radiation of the second wavelength, treating the layer of second photoresist material with the second solvent so as to develop a first periodic profile, exposing the layer of first photoresist material to radiation of the first wavelength through the first periodic profile and treating the layer of first photoresist material with the first solvent so as to develop a second periodic profile.
The use of dual layers of photoresist material in the process offers various advantages, including the ability to fabricate short pitch gratings which, because of the proximity of the mask formed in the second photoresist material, can be formed deep into the first photoresist material with a very high resolution. This mitigates some of the disadvantages associated with the chrome mask method described above.
Additional advantages of the dual layer photoresist process also include the masked areas of the layer of first photoresist material having minimal exposure to radiation of the first wavelength when the layer of first photoresist material is exposed to radiation of the first wavelength through the first periodic profile. This mitigates some of the disadvantage associated with the known technique of exposing a single layer of photoresist to an interference pattern.
A preferred embodiment of this invention provides a process for making a periodic profile in a photoresist layer wherein exposing the layer of first photoresist material to radiation of the first wavelength through the first periodic profile is performed with the radiation being directed toward the first layer at an angle away from the normal to the first photo-resist material. This allows blazed grating structures to be fabricated. A particular advantage of this embodiment is that it allows deep, short pitch blazed gratings to be made.
Additionally, the process for making a periodic profile in a photoresist layer described above may further comprise the initial step of depositing a layer of second photoresist on the layer of first photoresist and possibly depositing a layer of first photoresist on a substrate.
In a preferred embodiment, the process for making a periodic profile in a photoresist layer, as described above, makes use of PMGI as the first photoresist material and Shipley""s S1805 resist as the second photoresist material.